Problem: Which of these numbers is prime? ${6,\ 15,\ 67,\ 75,\ 99}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 6 are 1, 2, 3, and 6. The factors of 15 are 1, 3, 5, and 15. The factors of 67 are 1 and 67. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 67 is a prime number.